Data from: Female ground tits prefer relatives as extra-pair partners: driven by kin-selection?

Socially monogamous female birds routinely mate with males outside the pair bond. Three alternative hypotheses consider genetic benefits as the major driver behind the female strategy. The inbreeding avoidance hypothesis predicts that females paired with closely related males should seek copulations with distantly related extra-pair partners to avoid fitness loss from inbreeding depression; the outbreeding avoidance hypothesis predicts the opposite; the kin-selection hypothesis suggests that regardless of social mate relatedness, females should give related males extra-pair fertilization opportunities to gain inclusive fitness if the costs from inbreeding are minor. We test these hypotheses with a facultative cooperative breeder, the ground tit (Parus humilis). Social pairs of ground tits formed randomly with respect to genetic relatedness. In both bi-parental and cooperative groups, a female’s engaging in extra-pair mating was independent of relatedness to her social mate; however, females preferred extra-pair sires to which they were more related than to their social mates. Moreover, females had higher relatedness with either their extra-group extra-pair sires in both bi-parental and cooperative groups, or within-group helper sires in cooperative groups, than expected by chance. When more than one potential extra-pair partner was available around a female’s nest, she tended to select a relative. There was no indication of fitness reduction from extra-pair mating, which occurred at an intermediate level of inbreeding. These data support the kin-selection hypothesis, although there might be alternative non-genetic reasons associated with the extra-pair mating preference. Our finding offers a new explanation for why female birds pursue extra-pair mating. It also may broaden our understanding of the role of kin-selection in the evolution of cooperative society.实行社会一夫一妻制的雌性鸟类常会与配偶联结之外的雄性进行交配。现有三种备选假说将遗传收益视为雌性采取该配偶外交配策略的核心驱动因素。近交回避假说（inbreeding avoidance hypothesis）预测：若雌性与亲缘关系紧密的雄性配对，则会倾向于与亲缘关系较远的配偶外雄性交配，以规避近交衰退带来的适合度损失；远交回避假说（outbreeding avoidance hypothesis）则预测与之相反的结果；亲缘选择假说（kin-selection hypothesis）提出：若近交带来的成本较低，那么无论与社会配偶的亲缘关系如何，雌性都应为亲缘雄性提供配偶外受精机会，以获得广义适合度收益。 我们以兼性合作繁殖鸟类——地山雀（Parus humilis）为研究对象对上述假说进行了检验。地山雀的社会配对的形成与遗传亲缘关系无关，呈随机状态。无论是双亲家庭还是合作繁殖群体，雌性的配偶外交配行为均与其和社会配偶的亲缘关系无关；但相较于社会配偶，雌性更倾向于选择与自身亲缘关系更近的配偶外父本。 此外，无论是双亲家庭还是合作繁殖群体中的雌性，其与群外配偶外父本的亲缘关系，或是合作繁殖群体中与群内帮手父本的亲缘关系，均显著高于随机预期水平。当雌性巢穴周边存在多个潜在配偶外交配对象时，其往往会选择亲缘个体。本研究未发现配偶外交配带来适合度下降的证据，且该类交配发生时的近交程度处于中等水平。 尽管配偶外交配偏好可能存在其他非遗传层面的解释，但本研究数据支持亲缘选择假说。本研究发现为雌性鸟类采取配偶外交配策略提供了全新的解释视角，也有助于拓宽我们对亲缘选择在合作社会演化中所扮演角色的认知。